Known receivers use the correlation method to implement an optimum bit decision (i.e., whether a logic "1" or "0" was received). In such receivers, the received information signal is multiplied by a correlation pulse and their product is integrated for a specific period of time. However, practical IF filters, required to give the receiver acceptable selectivity, frequently cause severe distortion of the waveforms of the received signals. With such distortion, the correlation method does not work correctly because the resulting intersymbol interference causes errors in the received information or data. This signal distortion frequently results in an unacceptably high error rate for the receiver.
One known way to compensate for the effects of IF filter distortion is to predistort the information signal in the transmitter. This has two disadvantages. First, the same IF filter must be used in all receivers, thus limiting flexibility in the design of the receiver. Second, the predistortion may impart undesired characteristics to the transmitted signal, such as splatter and high peak amplitude.
Another known method is to incorporate gain/phase equalization into the receiver in an attempt to restore the original waveform of the signal (i.e., to undo the distortion) before the correlation operation. This practice, however adds substantial complexity to the receiver whether compensation is implemented in analog or digital form.